More toxic for Mitchell than being accused of swearing at police was the allegation that he called the officers "plebs", something he continues to deny.

Whereas Mitchell earned the nickname "Thrasher" due to his reputation for toughness, Sir George is seen as a much more courteous figure who is highly unlikely to get involved in any angry altercations with the police.

In his resignation letter published this evening, Mitchell said: "It has become clear to me that whatever the rights and wrongs of the matter I will not be able to fulfil my duties as we would both wish."

However he strongly denied using the words attributed to him: "I give you me categorical assurance again - that I did not, never have and never would call a police officer a 'pleb' or a 'moron' or used any of the other pejorative descriptions."

"The offending comment and the reason for my apology to the police was my parting remark 'I thought you guys were supposed to f***ing help us'.

"It was obviously wrong of me to use such bad language and I am very sorry about it and grateful to the police officer for accepting my apology."

In his reply, David Cameron said it was a matter of "regret" that he had resigned and praised him for the job he did as international development secretary.

"I am no doubt that your work in the field of international development has made a really important contribution," he said.

"You brought immense energy and dedication to the role, a focus on the world's poorest and decisive progress on the key issues of vaccination against preventable diseases, maternal health and family planning."

Sir George Young Promoted To Chief Whip After Andrew Mitchell's Resignation

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His resignation early on Friday evening came as chancellor George Osborne was embroiled in a new row over whether he had tried to travel first class on a train with only a standard class ticket.

The Labour Party has made great play of the perception that the Tory leadership is dominated by out of touch "posh boys" and the allegation that Mitchell used the word "pleb" re-enforced the damaging label.

Sir George Young Promoted To Chief Whip After Andrew Mitchell's Resignation

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Mitchell is the fourth coalition cabinet minister to have resigned, following Lib Dems Chris Huhne and David Laws and Tory defence secretary Liam Fox out the door.

A YouGov poll published last Friday showed 50% of public think the 'pleb' row showed that Tory MPs see themselves as better than ordinary people.

The survey revealed that 60% of the public believe Mitchell's outburst "showed his true feelings" and 50% think his behaviour and attitudes reflect those of other Conservative MPs.

Labour's Michael Dugher said: "David Cameron is left looking profoundly weak and totally out of touch, doing everything he could to hold on to Andrew Mitchell only for his Chief Whip to bow to the inevitable given the understandable public anger.

"There is a pattern of behaviour: an out of touch high handed government where the Chief Whip can insult the police as plebs and the Chancellor thinks he has a right to sit in first class without paying the fare.

He added: "This Tory government day by day show they think it’s one rule for them, another for everyone else.”